triTriGrid s returns a triangular grid with sides of
length s, using triangular tiles. If s is nonnegative, the
resulting grid will have s^2 tiles. Otherwise, the resulting grid
will be null and the list of indices will be null.

paraTriGrid r c returns a grid in the shape of a
parallelogram with r rows and c columns, using triangular
tiles. If r and c are both nonnegative, the resulting grid will
have 2*r*c tiles. Otherwise, the resulting grid will be null and
the list of indices will be null.

rectTriGrid r c returns a grid in the shape of a
rectangle (with jagged edges) that has r rows and c columns,
using triangular tiles. If r and c are both nonnegative, the
resulting grid will have 2*r*c tiles. Otherwise, the resulting grid will be null and
the list of indices will be null.

torTriGrid r c returns a toroidal grid with r rows and c
columns, using triangular tiles. The indexing method is the same as
for ParaTriGrid. If r and c are both nonnegative, the
resulting grid will have 2*r*c tiles. Otherwise, the resulting
grid will be null and the list of indices will be null.

A cylindrical grid with triangular tiles, where the cylinder is
along the y-axis.
The grid and its indexing scheme are illustrated in the user guide,
available at https://github.com/mhwombat/grid/wiki.

yCylTriGrid r c returns a cylindrical grid with r rows and
c columns, using triangular tiles, where the cylinder is along
the y-axis. The indexing method is the same as for ParaTriGrid.
If r and c are both nonnegative, the resulting grid will have
2*r*c tiles. Otherwise, the resulting grid will be null and the
list of indices will be null.

A cylindrical grid with triangular tiles, where the cylinder is
along the x-axis.
The grid and its indexing scheme are illustrated in the user guide,
available at https://github.com/mhwombat/grid/wiki.

xCylTriGrid r c returns a cylindrical grid with r rows and
c columns, using triangular tiles, where the cylinder is along
the y-axis. The indexing method is the same as for ParaTriGrid.
If r and c are both nonnegative, the resulting grid will have
2*r*c tiles. Otherwise, the resulting grid will be null and the
list of indices will be null.